The operating conditions of turbo-engines, especially aircraft engines, have led to the utilization of numerous titanium or titanium alloy components in such engines. It is important that these components should be subjected to a non-destructive checking capable revealing the various defects from which they may suffer. In particular, they should be examined for possible manufacturing defects such as segregations, inclusions, porosity, etc., transformation defects such as cracks, incrustations, heterogeneity, contaminations, etc., and machining or polishing defects such as work-hardening, local overheating, etc. For this purpose, there is in existence an electro-chemical etching process which is well known in the art as the "blue-etch process".
This electro-chemical etching process consists, generally, in carrying out the following operations on the component to be checked:
1. Conventional degreasing by immersion in an alkaline bath; PA1 2. Rinsing with cold water in a tank of running water, or by sprinkling; PA1 3. Possible removal of a work-hardened layer, about 5 microns, by fluo-nitric etching; PA1 4. Rinsing with cold water in a tank of running water; PA1 5. Chemical activation by immersion in an acid salt bath for etching with a macrographic effect; PA1 6. Rinsing with cold water in a tank of running water; PA1 7. Anodic oxidation in a trisodium phosphate bath, with the component to be checked being in the anode position; PA1 8. Rinsing with cold water in a tank of running water; PA1 9. Development by partial etching in a nitrohydrofluoric bath; PA1 10. Rinsing with cold water as quickly and thoroughly as possible, followed by drying of the component; and PA1 11. Reading the defects revealed, on the basis of shapes and colours (white, blue, grey-blue) which are peculiar to them.
However, this process does have some drawbacks. In particular, the nitrohydrofluoric development bath used in step 9 generally has a composition comprising, per liter, 320 g nitric acid (HNO.sub.3), from 13 to 22 g hydrofluoric acid (HF), and water the balance, and this requires the development to be carried out within a period of from 2 to 10 seconds, and the transfer time between the development bath and the rinsing of step 10 to be between 2 and 5 seconds. Exceeding one of these limits brings about complete discoloration of the component, making any detection of defects impossible.
As will be appreciated, it is not a problem to keep within these limits when treating components which are of relatively small size and simple shape, since these can be quickly handled and rinsed. However, this is not the case for relatively large components of complex shape, such as turbo-engine discs for example. It is therefore necessary, for such components, to reduce the activity of the developer bath so that the immersion and transfer times can be increased to be compatible with the process and handling equipment required for the components.
One way of reducing the reaction kinetics of the development bath is to reduce the concentration of the hydrofluoric acid in the bath. Unfortunately this solution results in a bath which becomes exhausted very quickly, and which therefore has a very short life and does not permit reliable results to be obtained.